The subject matter disclosed herein relates to an active flow control system and, more particularly, to an active flow control system for enhanced high-speed co-axial rotorcraft performance.
Gas flow in the shear layer adjacent to a surface exhibits a reduction in velocity due to friction of the molecular viscosity interacting with the surface, which results in a strong velocity gradient as a function of perpendicular distance from the surface: essentially zero at the surface and increasing to mainstream velocity at the outer edge of the boundary layer. The reduced velocity results in a lower momentum flux, which is the product of the density of the gas times the square of its velocity. Along a diverging surface (that is, a surface that tails away from the mean flow direction), as is the case on the suction side of an airfoil (such as a fan blade or helicopter blade), the flow along the surface is accompanied by a pressure rise, which is accomplished only by conversion of momentum flux. The momentum and energy of the gas along the surface is consumed in overcoming the pressure rise and friction so that the gas particles are finally brought to rest and the flow begins to break away from the wall resulting in boundary layer separation downstream of the separation point.
Boundary layer separation typically results in the termination of pressure rise (recovery) and hence loss in performance (e.g., airfoil lift). Boundary layer separation may also result in dramatic decreases in system efficiency due to conversions of flow energy into turbulence and eventually into heat.